Generally speaking, when traveling with a pokémon, always, always keep it as calm as possible. A baby crying is one thing. A raichu consuming all of the oxygen in a closed cabin and frying the electrical system with a single Thunderbolt is something entirely different. The best way to do this is to keep your poké ball on hand and be ready to recall your pokémon at the first sign of panic. In addition to that, bring plenty of distractions in the form of your pokémon’s favorite snacks and toys. Most importantly, interact with your pokémon throughout the ride. Show it signs of affection, talk to it, and play with it—anything you can do in a limited amount of space. The more you can distract your pokémon and show it that you’re calm, the calmer it will be.
Also, when the seatbelt sign is off, don’t be afraid to get up and walk your pokémon up and down the aisle (whenever possible, anyway). The exercise will likewise keep them calm and steady. However, try to avoid letting your pokémon roam without you, and never let your pokémon out of your sight. If you need to use the lavatory and your pokémon can’t fit in the cubicle with you, put it in its ball.
On that note, while takeoff isn’t so bad, landing can be rather alarming to a pokémon, especially due to the jarring bump at the end. Some pokémon can be calmed if they’re able to look out the window and watch the plane landing, but others would very, very much prefer an aisle seat. Usually, it’s easy to predict which your pokémon will be just by how they react to both heights and stress in general. The calmer a pokémon is, the more likely they won’t be affected by the view.
And of course, leading up to the trip, be sure to explain to your raichu as clearly as possible what the trip will entail. You might wish to consider showing him clips of planes in flight or images of the inside of an airplane, just so he can familiarize himself with what it means to travel.
Good luck and safe travels, anonymous!
The Pokédex According to Bill 